more POSIX compliance bugs

[Eric Blake]

The GNU extension of recognizing certain builtins only if arguments are passed 
is incompatible with other m4 implementations.  Also, POSIX only requires that 
the first argument of m4wrap be saved for later processing.  For example, in 
Solaris 8:

$ /usr/xpg4/bin/m4:
define
=>
m4wrap(a,b)
=>
^D
=>a

But with CVS head:
$ POSIXLY_CORRECT=1 m4:
define
=>define
m4wrap(a,b)
=>
^D
=>a b

While we can probably keep our m4wrap behavior as an extension, we should fix 
the behavior of expanding define into a null string when no arguments are 
present if POSIXLY_CORRECT or --traditional.

-- 
Eric Blake